The present invention relates generally to a seat belt tightening device (serving as a pretensioner) for eliminating the loosened state of a webbing which arises at the time of vehicle collision, and also to a seat belt apparatus having the same. More particularly, the present invention relates to improvement of a seat belt tightening device of the foregoing type including a driving unit wherein a piston displaceably received in a cylinder is driven by gas pressure with the aid of the driving unit so as to allow a certain intensity of tension to be applied to the webbing, and also to a seat belt apparatus having the same.
With a conventional seat belt apparatus operable for assuring safety of passengers sitting on a seat in a vehicle, since there is a tendency that an intensity of winding power given by a retractor is reduced for the purpose of attenuating uncomfortablity felt by each passenger when he puts on a webbing around his body, an extent of loosening of a seat belt caused when he puts on it about his body is unavoidably increased, resulting in effective and safe restricting of each passenger failing to be achieved due to the delayed tightening of the seat belt extending about his body.
In view of the foregoing malfunction of the conventional seat belt apparatus, a proposal has been made with respect to a seat belt tightening device incorporated in a seat belt apparatus wherein the seat belt tightening device is instantaneously retracted together with a buckle to assume the operative state by quickly pulling a webbing at the time of an occurrence of emergency such as vehicle collision or the like so as to allow the webbing to be tightened in order to eliminate the loosened state of the seat belt.
To facilitate understanding of the present invention, a typical conventional retractor having a pretensioner attached thereto will briefly be described below with reference to FIG. 9.
A retractor 240 shown in FIG. 9 includes a winding reel having a webbing 216 wound thereabout in the laminated structure in such a manner as to enable it to be quickly wound and unwound, and a pretensioner 243 serving as a seat belt tightening device for rotating a winding shaft for the webbing 216 in such a direction that the loosened state of a seat belt can be eliminated is arranged on one side wall of a retractor base 242 of the retractor 240 having a substantially U-shaped sectional contour. The pretensioner 243 serves to apply a certain intensity of tension to a wire 247 wound about a pulley (not shown) mounted on the winding shaft at one end of the latter wherein the wire 247 is used as a power transmission member. With this construction, the wire 247 is retractively driven at the time of an occurrence of emergency such as vehicle collision or the like in such a direction that the winding shaft is rotated in the webbing winding direction, whereby the webbing 216 extending about a passenger's body is quickly retracted in the retractor 240. To this end, the retractor 240 includes a driving unit 241 for applying a certain intensity of tension to the wire 247.
The driving unit 241 is composed of a piston 246 connected to one end of the wire 247, a cylinder 245 in which the piston 246 is slidably received, and a gas generator 244 connectably communicated with the cylinder 245. An ignition heater and an explosive are received in the gas generator 244. In response to a detection signal outputted from a collision sensor, the ignition heater is turned on to generate heat, causing the explosive to be exploded to generate gas. When the piston 246 is slidably driven in the cylinder 245 in the upward direction by the expansion pressure of the gas generated in the gas generator 244, the winding shaft is driven in the webbing winding direction so that the webbing 216 extending about the passenger's body is quickly retracted in the retractor 240, resulting in the loosened state of the seat belt being eliminated.
In addition to the aforementioned pretensioner, a mechanical type pretensioner is also put in practical use to serve in such a manner as to eliminate the loosened state of a seat belt by instantaneously retracting a buckle in the webbing stretching direction. This mechanical type pretensioner includes a weight adapted to move in response to an acceleration induced by shock and a trigger unit for releasing a compression spring from the preliminarily compressed state in operative association with the weight. With the mechanical type pretensioner constructed in that way, a webbing extending about a passenger's body is retracted in a driving unit by transmitting the restorative resilient power of the compression spring directly or indirectly to a buckle.
To raise up safety of each passenger with the conventional seat belt apparatus at the time of an occurrence of vehicle collision, it is necessary that the passenger is intensely restricted on his seat by intensifying the webbing tightening force of the pretensioner and increasing a quantity of webbing tightening activity. However, since the pretensioner utilizes the explosion power of the explosive and the resilient power of the compression spring as a driving power for the pretensioner, it is practically difficult to intensify the webbing tightening power and a quantity of webbing tightening activity with the pretensioner.
In case that the gas generator is used for the pretensioner as a driving power source for the webbing tightening activity, a quantity of explosive should be increased in order to intensify the webbing tightening power but the increasing of the quantity of explosive is strictly limited from the viewpoint of safe treatment of the explosive and safety of the pretensioner. To increase a quantity of explosive and intensify the gas pressure of the explosive, it is necessary that a container is disposed in the pretensioner for receiving the gas generator in the container with a sufficiently high strength against the pressure of the gas generated by explosion of an explosive. In this connection, a problem is that a production cost of the pretensioner is raised up due to the necessity for reinforcing other components rather than the container.
In case that the resilient power of the compression spring is utilized as a driving power for the pretensioner, to intensify the webbing tightening power, it is necessary that the compression spring is designed with large dimensions and a spring constant of the compression spring is increased. This leads to the result that other components such as a weight, a trigger unit or the like should additionally be reinforced with certain additional means. Thus, there arises a problem that a production cost of the pretensioner is undesirably raised up.
In addition, to assure that the webbing is more intensely tightened, it is generally required that the pressure of gas in the cylinder is elevated in the rear half of tightening time when the tension applied to the webbing in increased, in order to intensify the webbing tightening power for the seat belt tightening device. With the aforementioned driving unit having the explosion power of an explosive utilized therefor, however, it is practically difficult to properly control an intensity of webbing tightening power and a driving time of the webbing, because the gas pressure is quickly increased immediately after the explosive is exploded, and thereafter, it is gradually decreased. For this reason, it is difficult to obtain adequate webbing tightening properties with the seat belt tightening device, and moreover, it is difficult that the seat belt tightening device exhibits excellent passenger restriction performances. Similarly, with the aforementioned driving unit having the explosion power of the elastic force of the spring, it is practically difficult to properly control an intensity of webbing tightening power and a driving time of the webbing, because the elastic force of the spring is the highest at a setting position, and thereafter, it is gradually decreased.
The present invention has been made in consideration of the aforementioned background and its object resides in providing a seat belt apparatus including a plurality of inexpensive pretensioners wherein the seat belt apparatus makes it possible to easily increase an intensity of webbing tightening power and a quantity of webbing tightening activity, and moreover, it assures adequate webbing tightening performances wherein each passenger sitting on a seat in a vehicle can firmly be restricted with high safety.